Enable directory copying.

- Enable directory picking for copying.
- Implement recursive copying inside the CopyService.
  - Pretty up the notification (use an indeterminate notification while
    calculating copy size)
  - Do two recursive walks: one to determine the size of the copy job,
    and then another to actually copy the files.
- Switch to using ContentProviderClient instances, for better error
  detection and handling
- Disable copying from the Recents view.

Change-Id: Ieb38cca80edf84a487547b68f0d6b328fc4d7701